1. Field of the Invention
The present invention relates to an electric motor, in particular for high speed operation, comprising a housing; inside this housing, a stator having a core and windings toroidally wound on the core, with half-turns at the inside of the core and half-turns at the outside of the core; and inside the stator, a rotor.
2. Description of Related Art
In high speed motors the distribution of losses differs considerably from the distribution of losses in standard speed motors.
The speed of these high speed motors is commonly frequency controlled by an inverter. Due to harmonics coming from this inverter, the eddy current losses can easily become the dominant loss.
As these eddy current losses increase as the square of the design speed for the same frame size of the motor, these losses can be high compared with standard speed motors.
Moreover, as these losses have a tendency to be concentrated to the rotor, and cooling the rotor is most difficult, it is important to keep these eddy current losses as small as possible.
One known way to reduce these losses is to reduce the harmonic content from the inverter. This can be done by adding a filter between the inverter and the motor. In that way, the voltage supplied to the motor will have more the form of a sine wave.
If the eddy current losses are not magnitudes over what can be tolerated in the motor, said filter could be separate inductors outside the motor serially connected with said windings of said stator; those inductors need to be cooled, usually by means of convection cooling.
High speed motors are normally of very high power density and therefore often liquid cooled.
Separate standard convection cooled inductors outside the motor tend to be big and heavy for high power motors. They also add considerable cost to the drive system.
In a motor with toroidal windings, only the winding half-turns located at the inside of the core, facing the rotor, contribute to the generation of the main magnetic field and therefore to the generation of torque.
The stator winding half-turns at the outer side of said core generate a leakage flux and therefore form a part of the leakage inductance of the motor. This leakage inductance is not sufficient to reduce the harmonics due to inverter operation.